In animal ovaries, the development of maturational competence is a critical step during oocyte final differentiation. However, current knowledge of mechanisms regulating this event is extremely limited and inadequate. The ultimate goal of the proposed project is to determine the cellular and molecular mechanisms of hormonal regulation of oocyte final differentiation and maturation. The specific objective is to understand major mechanisms by which gonadotropin (GtH) induces maturational competence. The knowledge obtained would have clinical implications in the area of the physiology and regulation of gamete development and fertility. The target species is Xenopus laevis, since (1) this species is an important model in current studies on oocyte maturation, (2) its ovaries are a rich-source of ovarian follicles throughout the year which can be easily dissected into viable subfollicular compartments, and (3) much background information is available on the general and molecular biology of the Xenopus oocyte. The objective of this proposal will be accomplished by focusing on the following tasks: (1) to positively identify the natural amphibian (Xenopus) maturation-inducing steroid (MIS), (2) to examine the requirement for the production of MIS and other ovarian steroids during GtH induction of maturational competence, (3) to determine subfollicular sites of direct GtH action during the induction of maturational competence, (4) to examine molecular mechanisms associated with the GtH induction of maturational competence, and (5) to determine the effect of GtH on oocyte MIS receptor density as well as the mechanism for this effect during the induction of maturational competence. These questions remain unanswered or unclear (1-2) or have never been addressed before in any vertebrate species (3-5). Preliminary results are presented suggesting that, contrary to widespread assumptions, progesterone is not the amphibian MIS. If confirmed, this finding could significantly influence current concepts in the reproductive and molecular endocrinology of oocyte maturation. The experimental design for this proposal includes both physiological and biochemical (molecular) approaches, and all experiments will be performed under controlled in vitro conditions. The analytical tools include steroid biochemistry (chromatography and radioimmunoassay), germinal vesicle breakdown bioassays, radioreceptor assays, protein and RNA metabolic labeling, extraction, chromatography and electrophoresis, in vitro RNA translations.